Massive MIMO in Real Propagation Environments: Do All Antennas Contribute Equally?

Massive MIMO can greatly increase both spectral and transmit-energy efficiency. This is achieved by allowing the number of antennas and RF chains to grow very large. However, the challenges include high system complexity and hardware energy consumption. Here we investigate the possibilities to reduce the required number of RF chains, by performing antenna selection. While this approach is not a very effective strategy for theoretical independent Rayleigh fading channels, a substantial reduction in the number of RF chains can be achieved for real massive MIMO channels, without significant performance loss. We evaluate antenna selection performance on measured channels at 2.6 GHz, using a linear and a cylindrical array, both having 128 elements. Sum-rate maximization is used as the criterion for antenna selection. A selection scheme based on convex optimization is nearly optimal and used as a benchmark. The achieved sum-rate is compared with that of a very simple scheme that selects the antennas with the highest received power. The power-based scheme gives performance close to the convex optimization scheme, for the measured channels. This observation indicates a potential for significant reductions of massive MIMO implementation complexity, by reducing the number of RF chains and performing antenna selection using simple algorithms.Comment: Submitted to IEEE Transactions on Communication

Large antenna array and propagation environment interaction

In conventional MIMO, propagation conditions are often considered wide-sense stationary over the entire antenna array. In massive MIMO systems, where arrays can span over large physical dimensions, the situation is quite different. For instance, significant variations in signal strength, due to shadowing, can be experienced across a large array. These effects vary with propagation environment in which the array is placed, and influence achievable sum-rates. We characterize these variations for several measured propagation scenarios in the 2.6 GHz frequency range and illustrate how power variations and correlation properties change along the array